This application is related to Japanese Patent Application No. 2000-296671 filed in Sep. 28, 2000 and 2001-120011 filed in Apr. 18, 2000, whose priorities are claimed under 35 USC xc2xa7119, the disclosure of which is incorporated by reference in its entirety.
1. Field of the Invention
The present invention relates to a surface acoustic wave filter, and more particularly, it relates to a ladder type surface acoustic wave filter having plural surface acoustic wave resonators arranged in a series-arm and a parallel-arm.
2. Description of the Related Art
A ladder type surface acoustic wave (hereinafter referred to as SAW) filter using plural SAW resonators has been known as a band pass filter (see, for example, Journal of Institute of Electronics, Information and Communication Engineers A, Vol. J 76-A, No. 2, pp. 245-252 (1993)).
FIG. 12 shows a constitutional diagram of a conventional ladder type SAW filter.
A ladder type SAW filter has such a constitution that series-arm SAW resonators S1 an S2 are arranged between an input terminal Ti and an output terminal To of a piezoelectric substrate 1, and parallel-arm SAW resonators P1 and P2 are arranged between the input and output terminals and a ground terminal G. SAW resonators S1, S2, P1, P2, are generally called one-part SAW resonators.
FIG. 13 shows a constitutional diagram of a one-part SAW resonator. A one-part SAW resonator has such a constitution that an interdigital transducer (hereinafter referred to as IDT) 2 for electrically exciting SAW and reflectors 3-1 and 3-2 arranged on a propagation path of the SAW for trapping the excited SAW in the IDT are formed on the piezoelectric substrate 1.
The IDT 2 is formed with a large number of electrode fingers arranged in a comb form with a constant period (pi).
The reflectors 3-1 and 3-2 each is formed with a large number of grating electrode fingers 3-3 arranged with a constant period (pr) and is also referred to as a grating reflector.
In the IDT, the SAW is excited by two adjacent electrode fingers each extending downward and upward as one unit.
A ladder type SAW filter is designed in such a manner that a resonance frequency fr of the series-arm SAW resonators S1 and S2 substantially agrees to an antiresonance frequency fa of the parallel-arm SAW resonators P1 and P2.
FIG. 14(a) is a graph showing a pass characteristic diagram of a ladder type SAW filter, and FIG. 14(b) is a graph showing single impedance characteristic diagrams at this time of the series-arm SAW resonators S1 an S2 and the parallel-arm SAW resonators P1 and P2. The graph g1 in FIG. 14(b) is an impedance characteristic of the series-arm SAW resonators S1 an S2, and the graph g2 is an impedance characteristic of the parallel-arm SAW resonators P1 and P2. In the graph g1 of the series-arm SAW resonators, the frequency where the impedance becomes minimum is the resonance frequency frs, and the frequency where the impedance becomes maximum is the antiresonance frequency fas.
FIG. 15 is an explanatory diagram of frequency characteristics demanded for a band pass filter, such as a ladder type SAW filter.
The characteristic values herein include demanded pass band widths (BW1 and BW2), attenuation level (ATT1 and ATT2) defined by the specification, and attenuation band widths (BWatt1 and BWatt2).
A ratio (BW1/BW2) of the band widths BW2 and BW1 at a certain attenuation level is referred to as shape factor. The closer the shape factor to 1, it is better and referred to as a high shape factor. The shape factor of the ladder type SAW filter is substantially decided by the frequency difference between the resonance frequency fr and the antiresonance frequency fa.
That is, the steepness of the inclination from the attenuation region on the lower frequency side to the pass region depends on the frequency difference (xcex94fp in FIG. 14(b)) between the resonance frequency frp and the antiresonance frequency fap of the parallel-arm resonators P1 and P2, and the smaller the xcex94fp is, the steeper the inclination is. The steepness of the declination from the pass region to the attenuation region on the higher frequency side depends on the frequency difference (xcex94fs in FIG. 14(b)) between the resonance frequency frs and the antiresonance frequency fas of the series-arm resonators S1 and S2, and the smaller the xcex94fs is, the steeper the declination is.
However, the xcex94fp and the xcex94fs are substantially decided by the electromechanical coupling factor of the used piezoelectric substrate 1, and do not vary by changing the number of pairs of the electrode fingers and the aperture length of the IDT. In this connection, Japanese Unexamined Patent Publication No. Hei 11(1999)-163664 discloses a SAW filter having xcex94fp and xcex94fs that are made small by periodically withdrawing the electrode fingers of the IDT. The xcex94fp and the xcex94fs can be adjusted with a simple constitution by using the withdrawing method, so as to realize a ladder type SAW filter having a desired pass band width and steepness of the edge parts of the pass region.
Furthermore, Japanese Unexamined Patent Publication No. 2000-315931 discloses a SAW resonator having electrode fingers connected to a positive potential and a negative potential that are inverted each other, so as to increase the steepness of the filter characteristics in the vicinity of the pass band.
However, in the case where the electrode fingers of the IDT are periodically withdrawing, both the xcex94fp and the xcex94fs become small, and there is a problem in that the capacitance of the IDT is decreased by the withdrawing although a filter having a high shape factor can be realized.
The decrease in capacitance of the IDT brings about mismatch of input and output impedance, which adversely affects the characteristics of the SAW filter.
In general, enlargement of the aperture length of the IDT or increase of the number of pairs of the electrode fingers of the IDT is conducted to compensate the decrease in capacitance of the IDT. However, the enlargement of the aperture length increases the size of the IDT, which restricts the demand in miniaturization of the filter chip size made upon utilization in portable phones.
For example, when 20% of the electrode fingers are withdrawed from the whole electrode fingers of the IDT, the capacitance of the IDT is decreased by 40%, which brings about enlargement of the IDT size for compensating the capacitance of about 1.67 times. This does not match the demanded specification of realizing a compact SAW filter.
On the other hand, when the electrode pair is inverted, the decrease in capacitance of IDT can be suppressed low, and the xcex94fp and the xcex94fs can be made small, so as to realize a SAW filter of a high shape factor having a smaller size than those made by the withdrawing method. However, it is necessary that the positions where the inverted regions are provided are carefully considered as in the withdrawing method. Because when the electrode pair is inverted periodically, there is a high possibility that plural numbers of spurious occur outside the pass band of the filter, so as to fail to satisfy the demanded specification. Furthermore, when several numbers or more of the electrodes are continuously inverted, the effect of minifying the xcex94fp and the xcex94fs is impaired to bring about necessity of increasing the number of the inverted regions, and as a result the size of the filter is increased.
The invention relates to a surface acoustic wave filter comprising a piezoelectric substrate having formed thereon a plurality of surface acoustic wave resonators electrically connected in a ladder form for exciting a surface acoustic wave, wherein each of the surface acoustic wave resonators is composed of an interdigital transducer and reflectors arranged closely on both sides of the interdigital transducer in a direction parallel to a propagation direction of the excited surface acoustic wave; the interdigital transducer of at least one of said surface acoustic wave resonators is composed of a prescribed number of pairs of comb form electrodes; and the surface acoustic wave excited by the pairs of comb form electrodes contains two kinds of surface acoustic waves having phases that are different from each other by 180xc2x0.
According to the invention, the resonance frequency and the antiresonance frequency of the surface acoustic wave resonator can be made to approach each other, and the shape factor of the surface acoustic wave filter can be increased, whereby a compact surface acoustic wave filter in that decrease in capacitance of the IDT is suppressed.